1. Field of the Invention
The present invention relates to vapor-liquid contact packing and, more particularly, to corrugated contact plates disposed in face-to-face contact for use in vapor-liquid process towers.
2. History of the Prior Art
In the vapor-liquid contact art, it is highly desireable to utilize methods and apparatus that efficiently improve the quality as well as the quantity of the mass heat transfer occurring in process towers. The technology of such process towers is replete with material designs used for tower packing. The type of packing is a function of the particular process to be effected within the tower. The packing elements may comprise a structured grid array (grid packing) arranged to form a regular array inside the column or may comprise oblique shapes dumped into and randomly arranged (dump packing) within the tower. Close fractionation and/or separation of the feed stock constituents introduced into the tower and the elimination of harmful or undesirable residual elements imparts criticality to the particular vapor-liquid contact apparatus designed. The shape of the dump packing elements determines the flow pattern in and density of the array and the resultant resistance to flow caused thereby. Prior art grid arrays have thus found utility in a variety of shapes, sized and material forms in both structure arrays and dump packing configurations.
It has been found particularly desirable in the prior art to provide apparatus and methods affording efficient heat transfer and fluid vaporization, or vapor condensing whereby cooling of one of the fluids can be accomplished with a minimum pressure drop through and in a zone of minimum dimensions defining its area and volume. High efficiency, low pressure drop and reduced temperatures are most often found as design criteria in the chemical engineering art particularly applied to petroleum refraction operations. Process towers for effecting such chemical reactions are generally of the character providing descending fluid flow from an upper portion of the tower and ascending vapor flow from a lower portion of the tower. Sufficient surficial area for vapor-liquid contact is necessary for the primary function and the reduction or elimination of liquid entrainment present in the ascending vapor. Most often it is necessary for the grid array to have sufficient mass and surficial area in both its horizontal and vertical planes so that fractions of the heavy constituents are conducted downwardly in condensed form and the vapors are permitted to rise through the grid with minimum impedence. With such apparatus, undesirable solids or heavy constituents of the feed stock are removed by the coaction of the ascending liquid vapor to provide a self-cleaning grid.
Generally, a plurality of stacked layers affording compatible and complemental design configurations for a particular application are assembled within a single column. Each layer utilizes the velocity and kinetic energy of the ascending vapors to perform the dual function of eliminating liquid entrainment in the ascending vapor and the thorough and turbulent contacting of the vapor with the descending liquid to accomplish sufficient separation, or fractionation, of the fluids into the desired components. Quick cooling of the ascending vapor is generally a prerequisite for efficient operation to effect efficient heat transfer for vapor condensation and minimum pressure drop in a minimum vertical depth of the grid. Oppositely inclined corregated plates have thus been utilized in the prior art for affording multiple vapor passages through the horizontal and vertical planes of the grid layers. Such complex flow patterns insure the flow of vapors and the distribution thereof within the layers which prevents maldistribution or a channeling of the vapor through only certain portion of the layers and not others. Only in this manner is efficient and effective utilization of the column and the energies applied therein effected.
Prior art structures often incorporate a plurality of layers with the grid members of each layer having angularly disposed elements in contiguous contact. Each element generally has a structural configuration and angularity that permits a large upright vapor passage area in excess of fifty percent of the horizontal area of the layer. This design usually affords acceptable efficiency and vapor-liquid distribution for heat mass transfer. Such structures also by necessity provide thorough and turbulent mixing or contacting of ascending vapor and descending liquid without materially displacing either the vapor or liquid from its vertical location or flow within the grid. Such displacement would cause maldistribution or channeling of either the vapor or the liquid through certain portions of the grid or its layers, which would reduce efficiency.
The structural configuration of oppositely inclined corrugated plates of the prior art often incorporate vapor passages such as plate orifices whereby turbulence is enhanced. The orifices insure intimate vapor-liquid contact and are often comprised of simple holes punched in the plates. It is nececessary to insure the acsending vapor performs a dual function of liquid contact and liquid disentrainment within close proximity to the vertical location at which the ascending vapor approaches or leaves the vapor passage orifices. In this manner, maldistribution of the ascending vapor or descending liquid is prevented. It is, moreover, of tantamount concern in the prior art to provide such methods and apparatus for vapor-liquid contact in a configuration of economical manufacture. Such considerations are necessary for cost effectiveness.
Oppositely inclined corrugated plates provide but one method and apparatus for countercurrent, liquid-vapor interaction. With such grid arrays, the liquid introduced at or near the top of the column and withdrawn at the bottom is brought into contact with vapor being introduced at or near the bottom of the column and withdrawn at the top. The critical feature in such methods and apparatus is to insure that the liquid and vapor achieve the desired degree of contact with each other so that the planned reaction occurs at the designed rate within controlled parameters of mass and heat transfer. The internal structure is, of course, passive in the sense that it is not power driven and has few or no moving parts. The prior art is thus replete with such passive vapor-liquid contact devices utilizing cross-fluted and perforated sheets of material in face-to-face engagement for encouraging the liquid moving through to form itself into thin films. The films have, in the aggregate, a large area over which to pass for the vapor flowing through the corrugations to engage. But the design problem is not merely a matter of providing a large surface area or a multitude of corrugations, cross-flutes, or perforations. A number of other interrelated considerations must be taken into account, some of which have been mentioned above, but which determine operational efficiency and operational effectiveness.
From a process standpoint, it is important that the desired vapor-liquid contact reaction be carried as close to completion as possible. For example, in a crude oil vacuum tower, close fractionation and good separation are needed to produce gas oil streams that are free of undesirable residual elements. As mentioned above, the contact column in its internal apparatus must thus utilize the heat supplied to the unit efficiently. In this manner it minimizes direct operating costs. This is true whether the reaction is mass transfer, heat transfer, liquid-vaporization or vapor condensing duty. With the above considerations, pressure drop is a primary consideration as is the vapor-liquid fluid interface. Such grids for vapor-liquid contact have been shown in the prior art in such references as U.S. Pat. No. 3,343,821, issued Sept. 26, 1967; U.S. Pat. No. 4,139,584, issued Feb. 13, 1979; U.S. Pat. No. 4,128,684, issued Dec. 5, 1978; U.S. Pat. No. 3,785,620, issued Jan. 15, 1974; and U.S. Pat. No. 3,959,419, issued May 25, 1976. In these vapor-liquid contact method and apparatus references, a plurality of design configurations are presented for affording intimate vapor-liquid contact. In particular, stacked corrugated contact plates in face-to-face contact having corrugations inclined to the horizontal and/or orthogonal one to the other have been shown. These plates have also been provided in various material configurations, including monofilament yarns, and solid plates. It is moreover prominent in the prior art to utilize cross-fluted plates having a myriad of perforations therethrough for improved effectiveness.
While the methods and apparatus set forth above for vapor-liquid contact have been shown to be effective, certain disadvantages yet remain. In particular, vapor-liquid contact towers incorporating descending liquid flow and ascending vapor flow of the passive grid variety defined above, is generally incapable of self-regulation of internal pressure differentials. Moreover, non-homogenous vapor-liquid flow across the grid surface area is prevalent and leads to a reduction in mass heat transfer and operational efficiency. Even with a plurality of apertures disposed between substantially planar and/or cross-fluted plates of the prior art variety, vapor flow is ultimately sensitive to pressure differentials.
Pressure differentials in process columns are often imparted due to non-turbulent vapor flow and non-homogenous flow patterns through the grid structure. When vapor flow is laminar through a first fluted column and turbulent in an area around a second adjacent fluted column pressure differentials are imparted. Even when the corrugations or cross-fluted areas of adjacent plates are inclined to relatively sharp angles, one to the other, vapor flowing along any one corrugation is exposed along over fifty percent of the sidewall having substantially solid surface and a plurality of major openings defining flow channels along the second wall section thereof. Turbulence in an adjacent section, therefore, directly affects the laminar flow and imparts pressure differentials. Turbulence along all channels and all adjacent openings provides uniformity and less propensity for fluid displacement and maldistribution or channeling of either the vapor or the liquid through certain portions of the grid or its layers. Such dynamic action directly affects efficiency and mass heat interaction.
Vapor and liquid flow in the above prior art configurations has been shown to be susceptible to random flow patterns which cannot be accurately determined within the passive grid of most prior art designs. The absence of any substantial degree of uniformly imparted turbulence through the adjacent corrugations and fluted areas decreased the uniformity and homogeneity of the flow pattern throughout the grid and the programmed efficiency and mass heat transfer characteristics capable of predefined grid structures with known flow characteristics. Moreover, the utilization of random apertures between face-to-face corrugations and/or fluted plates has limited effectiveness due to the planar liquid and vapor flow adjacent thereto. The apertures are, in effect, planar voids within a substantially planar flow area which imparts little direct turbulence to either the vapor or liquid. For this reason, packing elements made of foil-like material having alternating smooth and finely fluted portions such as that shown in U.S. Pat. No. 4,186,159 have been utilized in the prior art. The packing elements set forth in the above patent incorporate a plurality of corrugated plates spirally wound from a continuous strip to form an ordered packing with a plurality of apertures formed therethrough. The flow distribution is improved along the cross-fluted areas but not uniformly across the entire surface of the corrugated plate. Moreover, the fluid flowing on one side is substantially confined to a single side of the plate rather than being dispersed through to the other side as would be advantageous in maximum efficiency and maximum exposure of the liquid to a vapor flow in a turbulent region.
It would be an advantage, therefore, to overcome the problems of the prior art by incorporating the advantages of face-to-face corrugated and fluted contact plates with the utilization of a plate material imparting both vapor and fluid turbulence and fluid vapor interchange for maximizing efficiency. The expanded metal packing and method of manufacture set forth in the present invention provides such a packing with enhanced vapor-liquid contact without adversely affecting the operational characteristics or adding to pressure losses therethrough. The methods and apparatus of the present invention provide such an improvement over the prior art grid by providing an expanded metal plate of twisted lands defining a myriad of aperatures therebetween in a corrugated configuration. The corrugated plates are assembled in face-to-face relationship with the corrugation angle facing opposite directions along the notional separation plane therebetween. The presence of the twisted metal lands defining the apertures therebetween permit the formation of turbulent liquid flow thereover and through the plate whereby both sides are substantially filmed over by the descending liquid flow. The ascending vapor flow is further enhanced by the turbulence imparted thereto by the corrugated expanded metal. Such vapor-liquid flow configurations are, in effect, maximum utilization of process tower technology. This is made possible by providing a myriad of narrow, twisted flow channels for the fluid to be dispersed along through opposite sides of the corrugated plates. The expanded metal affect such flow while simultaneously imparting turbulence to the otherwise laminar vapor flow along the plate, whereby the vapor and fluid are induced to flow along and through the expanded metal surface for maximum exposure of the surface areas of both.